diff --git a/.env.example b/.env.example
index 2fb62a7..5df4ac3 100644
--- a/.env.example
+++ b/.env.example
@@ -133,6 +133,32 @@ PRICES_FETCH_INTERVAL_SECS=300
 # Tips are parked for the conditional-transfer redesign; off until that lands.
 FEATURE_TIPS=false
 
+# ── Nostr relay (optional messaging plane; see docs/operations/RELAY.md) ────
+# Off by default. When on, the operator runs a relay (the packaged
+# nostr-rs-relay under deploy/relay, or an external one) that this backend
+# advertises (/capabilities + NIP-05) and write-restricts via a gRPC admission
+# service. It is the user's DM INBOX, never the only relay — clients also use the
+# public interop relays. Self-hosting concern.
+RELAY_ENABLED=false
+# bundled (run deploy/relay's nostr-rs-relay) | external (point at your own).
+RELAY_MODE=bundled
+# Public ws(s):// URL clients connect to (wss required in production).
+RELAY_URL=
+# Write policy the admission service enforces:
+#   inbox-outbox     — registered npubs publish their own events; anyone may
+#                      deliver a gift-wrapped DM (kind 1059) to a registered user.
+#   author-allowlist — only registered npubs may publish anything.
+#   open             — accept everything (relay resource caps still apply).
+RELAY_WRITE_POLICY=inbox-outbox
+# NIP-13 proof-of-work bits required on cross-ecosystem (non-registered-author)
+# inbound events; 0 disables the PoW gate.
+RELAY_INBOUND_POW_BITS=0
+# Loopback address the gRPC event-admission service binds (the relay calls it).
+# Keep it loopback; the bundled compose reaches it via the host network.
+RELAY_ADMISSION_BIND=127.0.0.1:8090
+RELAY_MAX_EVENT_BYTES=65536
+RELAY_RETENTION_DAYS=30
+
 # ── Admin surface (see docs; default posture is loopback/Tor) ──────────────
 # Admin auth is "Sign in with Smirk": only admin PUBLIC keys are stored here,
 # never a seed or private key. The allowlist is mutated CLI/loopback-only in v1.
diff --git a/deploy/relay/config.toml b/deploy/relay/config.toml
new file mode 100644
index 0000000..f62b002
--- /dev/null
+++ b/deploy/relay/config.toml
@@ -0,0 +1,50 @@
+# nostr-rs-relay config for a locked-down Smirk messaging relay.
+#
+# REFERENCE TEMPLATE — the exact key set is version-specific; check it against the
+# nostr-rs-relay release you pin. The load-bearing piece is [grpc]
+# event_admission_server: it points at the Smirk backend's gRPC admission service,
+# which enforces RELAY_WRITE_POLICY + the NIP-13 PoW gate per event. Do NOT also
+# set a static [authorization] pubkey_whitelist — that would double-gate and
+# break the inbox (gift-wraps from non-registered senders).
+
+[info]
+relay_url = "wss://relay.example.org/"
+name = "Smirk relay"
+description = "Locked-down Nostr relay for a self-hosted Smirk backend (DM inbox)."
+# pubkey = "<operator x-only hex pubkey>"   # optional contact identity
+
+[database]
+# SQLite is simplest for a single-node self-host; switch to postgres to share
+# infra. Keep it on a persisted volume (see docker-compose.yml).
+engine = "sqlite"
+data_directory = "/usr/src/app/db"
+
+[grpc]
+# Smirk backend event-admission (nauthz). With host networking the backend's
+# loopback RELAY_ADMISSION_BIND is reachable here. This is what enforces the
+# write policy — keep it set.
+event_admission_server = "http://127.0.0.1:8090"
+
+[network]
+address = "0.0.0.0"
+port = 8080
+
+# Conservative, self-host-friendly resource ceilings (the anti-abuse floor even
+# for the `open` policy). Tune to your box.
+[limits]
+messages_per_sec = 10
+subscriptions_per_min = 60
+max_event_bytes = 65536
+max_ws_message_bytes = 131072
+max_ws_frame_bytes = 131072
+broadcast_buffer = 16384
+event_persist_buffer = 4096
+limit_scrapers = true
+
+[options]
+# Reject events dated too far in the future (clock-skew abuse).
+reject_future_seconds = 1800
+
+[authorization]
+# Write restriction is enforced by the gRPC admission service above, NOT a static
+# whitelist. Leave pubkey_whitelist unset.
diff --git a/deploy/relay/docker-compose.yml b/deploy/relay/docker-compose.yml
new file mode 100644
index 0000000..4c26ebd
--- /dev/null
+++ b/deploy/relay/docker-compose.yml
@@ -0,0 +1,35 @@
+# Bundled, locked-down Nostr relay for a self-hosted Smirk backend.
+#
+# This runs nostr-rs-relay as the messaging-plane relay. WRITE ADMISSION is
+# delegated to the Smirk backend's gRPC service (RELAY_ADMISSION_BIND, default
+# 127.0.0.1:8090) via nostr-rs-relay's [grpc] event_admission_server hook — so
+# the RELAY_WRITE_POLICY (inbox-outbox / author-allowlist / open) + NIP-13 PoW
+# are enforced by the backend, not baked into this file.
+#
+# Networking: the relay uses the HOST network so it can reach the backend's
+# loopback admission socket (127.0.0.1:8090) while keeping that socket private.
+# host networking is Linux-only; on other hosts, bind RELAY_ADMISSION_BIND to a
+# docker-reachable address behind your firewall and drop `network_mode: host`,
+# adding `extra_hosts: ["host.docker.internal:host-gateway"]` + a port mapping.
+#
+# This is a REFERENCE template — verify the config keys against the nostr-rs-relay
+# version you pin (https://github.com/scsibug/nostr-rs-relay).
+services:
+  relay:
+    image: ghcr.io/scsibug/nostr-rs-relay:latest # pin a digest in production
+    restart: unless-stopped
+    network_mode: host # reach the backend admission socket on 127.0.0.1
+    user: "1000:1000"
+    volumes:
+      - ./config.toml:/usr/src/app/config.toml:ro
+      - relay-data:/usr/src/app/db
+    environment:
+      RUST_LOG: info
+    # Modest, self-host-friendly resource ceilings.
+    deploy:
+      resources:
+        limits:
+          memory: 512M
+
+volumes:
+  relay-data:
diff --git a/docs/operations/OPERATOR_SETUP.md b/docs/operations/OPERATOR_SETUP.md
index b095d78..902fbbe 100644
--- a/docs/operations/OPERATOR_SETUP.md
+++ b/docs/operations/OPERATOR_SETUP.md
@@ -110,6 +110,13 @@ self-hosting bypass them. All default off or open.
 - **Restore policy** — `WALLET_RESTORE_POLICY=create-only|bounded|unlimited` (default `create-only`), with optional proof-of-work pricing on restore depth.
 - **Pay-to-register** — `REGISTRATION_REQUIRE_PAYMENT=true` against a BTCPay-compatible processor (`PAYMENT_*`); a new wallet settles an invoice, paid to your own wallet, before it can register.
 
+## 7. Nostr relay (optional)
+
+Run a first-party, locked-down Nostr relay as your users' encrypted-DM inbox
+(`RELAY_ENABLED=true` + `RELAY_*`). Off by default; write-restricted via a gRPC
+admission service, advertised through `/capabilities` + NIP-05. Full guide:
+[RELAY.md](RELAY.md).
+
 ## Testing an instance
 
 The `@smirk/smoke-tests` harness drives the API client end-to-end. Point it at an
diff --git a/docs/operations/RELAY.md b/docs/operations/RELAY.md
new file mode 100644
index 0000000..769e08c
--- /dev/null
+++ b/docs/operations/RELAY.md
@@ -0,0 +1,87 @@
+# Nostr relay (messaging plane) — operator guide
+
+The relay is **optional**. It gives your users a first-party, **locked-down**
+Nostr relay that acts as their **encrypted-DM inbox** — the transport foundation
+for messaging (and, later, P2P coordination). It is **off by default**; a Smirk
+backend is fully functional without it.
+
+## What it is (and isn't)
+
+- **It is the user's inbox relay**, advertised via `/capabilities` and NIP-05
+  relay hints, and **write-restricted** by policy (see below).
+- **It is NOT the only relay.** For cross-wallet delivery (e.g. to/from Goblin or
+  any Nostr user), clients also connect to the public interop relays
+  (`relay.damus.io`, `nos.lol`). A private relay alone would black-hole DMs to
+  outsiders — so the NIP-05 hints list your relay **first**, then the public set.
+- The backend **does not run the relay process** — it *connects to + advertises*
+  it and enforces the write policy. You run the relay (bundled or external).
+
+## Architecture
+
+```
+wallet ──ws──▶ your relay (nostr-rs-relay) ──gRPC EventAdmit──▶ smirk-backend
+                                                                (RELAY_WRITE_POLICY
+                                                                 + NIP-13 PoW,
+                                                                 registered-npub DB)
+```
+
+`nostr-rs-relay`'s `[grpc] event_admission_server` calls the backend once per
+inbound event; the backend runs the pure policy engine (`src/infra/relay/policy.rs`)
+against the user DB and returns PERMIT/DENY. This is dynamic (per-event, per-
+recipient), which a static pubkey whitelist can't do.
+
+## Write policies (`RELAY_WRITE_POLICY`)
+
+| policy | who may publish |
+| --- | --- |
+| `inbox-outbox` (default) | registered npubs publish their own events (outbox); **anyone** may deliver a NIP-17 gift-wrap (kind 1059) **addressed to a registered user** (inbox) |
+| `author-allowlist` | only registered Smirk npubs may publish anything (no external inbound) |
+| `open` | accept everything (the relay's resource caps still apply) |
+
+`RELAY_INBOUND_POW_BITS` (0 = off) requires a NIP-13 proof-of-work on
+cross-ecosystem (non-registered-author) inbound events — spam friction without an
+author allowlist. Only the `inbox-outbox` external-inbound branch is gated.
+
+## Enable it
+
+1. Set the relay env (see `.env.example` → *Nostr relay*):
+   ```sh
+   RELAY_ENABLED=true
+   RELAY_MODE=bundled                    # or external
+   RELAY_URL=wss://relay.yourdomain.tld  # what clients connect to (wss in prod)
+   RELAY_WRITE_POLICY=inbox-outbox
+   RELAY_INBOUND_POW_BITS=0
+   RELAY_ADMISSION_BIND=127.0.0.1:8090   # loopback gRPC admission
+   ```
+   The backend fail-closes at startup on a bad relay config, and for any non-`open`
+   policy it spawns the gRPC admission service on `RELAY_ADMISSION_BIND`.
+
+2. **Bundled** (`RELAY_MODE=bundled`): run the reference `nostr-rs-relay` in
+   [`deploy/relay/`](../../deploy/relay/):
+   ```sh
+   # set relay_url + event_admission_server in deploy/relay/config.toml first
+   docker compose -f deploy/relay/docker-compose.yml up -d
+   ```
+   It uses host networking so the relay reaches the loopback admission socket
+   while that socket stays private (Linux; see the compose comments for other
+   hosts). Front `RELAY_URL` with your TLS terminator.
+
+3. **External** (`RELAY_MODE=external`): point `RELAY_URL` at any relay you run,
+   and configure *its* `[grpc] event_admission_server` at `RELAY_ADMISSION_BIND`.
+
+## Verify
+
+- `GET /capabilities` → `features.nostr_relay: true` and a `messaging` object
+  (`relay_url`, `write_policy`, `inbound_pow_bits`, `supported_nips`).
+- `GET /.well-known/nostr.json?name=<user>` lists your relay first in `relays`.
+- Publish a registered-npub event → accepted; a random-author, non-gift-wrap
+  event → rejected (`inbox-outbox`); an under-PoW cross-ecosystem event →
+  rejected (when `RELAY_INBOUND_POW_BITS > 0`).
+
+## Modularity
+
+`nostr-rs-relay` is the first adapter behind the `RelayProvider` seam
+(`src/infra/relay/`). The write policy lives in a pure engine; the gRPC transport
+is the adapter's concern. A different relay (strfry, or another messaging
+protocol) can enforce the same policy over a different mechanism without touching
+the policy or the advertising.